Long-Term Objectives: Determination of the programmed and environmental controls operating in the differentiation of normal skeletal muscle cells. Applicability of regeneration, aging. Specific Aims: Testing of predictions and general validity of a lineage model of myogenesis both in vitro and in vivo. In particular: 1. Isolation and characterization of the myogenic stem cell. a. Determination of which proteins or active genes are unique to this cell. b. Determination of the environmental controls that affect the types of progeny produced by this cell. c. Determination of whether this cell decides what types of proteins are to be synthesized at the end of the lineage. d. Determination of the relationship of this cell to muscle fibroblasts. 2. Testing the lineage model in vivo in the chick embryo. 3. Testing the model in other species. 4. Identification of the lineage stage-specific markets (genetic and/or protein) and markers for non-myogenic cells. 5. Determination of chromatin structural changes as cells traverse the lineage. Methodology: Tissue culture, protein purification, polyclonal and monlclonal antibody production and characterization, autoradiography, cell fractionation by density gradient centrifugation, cloning of chick embryo precursor cell genes (cDNA's), immunocytochemistry.